Ground glass joints have been used for years to join separate pieces of glassware. In such joints, the mating surfaces of the pieces to be joined are precision ground to fit closely one against the other, so as to create a joint that is as impermeable as possible.
One particularly useful joint is the ball joint, which is typically used to releasably join two pieces of glass tubing. To create the ball joint, a glass ball is fashioned on the mating end of one of the glass tubes. The exterior surface of the free end of the glass ball is precision ground to create a very uniform surface. A glass socket is fashioned on the mating end of the other glass tube. The interior surface of the free end of the glass socket is also precision ground, to create a uniform surface having a diameter that is only slightly larger than the diameter of the ground glass ball. A joint is created between the two pieces of glass tubing when the ball is fitted within the socket.
One benefit of the ball joint is that the two glass tubes do not have to remain in an axially aligned position relative to each other. This is possible because the ball on the end of the one tube can rotate to a certain degree within the socket on the end of the other tube, while the joint created between the ball and socket remains intact.
Although the precision ground ball and socket fit very well together in the arrangement described above, the joint may not be sufficiently impermeable for all applications. Thus, some type of seal is often placed within the joint in some manner. For example, a small amount of water or vacuum grease can be used to coat the ground surfaces of the ball and socket, which tends to increase the impermeability of the joint for certain applications.
However, sealants of the type mentioned above are inappropriate for other applications. For example, if the joint must be held at or cycled to an elevated temperature, water will tend to evaporate from the joint, and will no longer function as a sealant. At even higher temperatures, the same will happen with grease. Further, because they tend to be somewhat volatile at almost any practical temperature, these types of sealants may be a source of contamination to the fluid flowing through the joined glass tubes. In addition, even if the volatility of these sealants is not a significant problem, they may be chemically reactive with the fluid flowing through the tubes. As used herein, the word "fluid" means "anything that flows." Thus, "fluid" comprehends, without limitation, gases, liquids, suspensions, flowable solids, and mixtures of the same.
For applications in which these types of sealants are inappropriate, an o-ring can be used. Because the o-ring is typically made of a deformable solid and not a liquid or thixotrope, o-rings tend to be less volatile and reactive than the sealants mentioned above. Typical materials for o-rings include natural and synthetic rubbers and other thermoplastic resins, such as Viton and Buna-N. When an o-ring is used, either the socket, or more preferably, the ball is modified with an annular channel to receive and hold the o-ring.
Unfortunately, some applications, such as higher temperature applications and applications in which a fluid that is corrosive to the seal is flowing through the joint, exceed the ability of the o-ring to resist degradation. For example, at a sufficiently high temperature for a sufficient length of time, most o-ring materials will loose some amount of their more volatile components.
This degradation of the o-ring compromises the integrity of the joint seal, which can adversely affect the process in which the joint is used. For example, if the static or dynamic pressure within the glass tubes is less than the pressure outside of the joint, then the loss of the o-ring seal may allow air or other fluids from outside of the joint to be drawn into the joint, thus increasing the pressure within the glass tubes. Additionally, the loss of the seal may allow the environment on the outside of the joint to permeate the joint and contaminate the environment inside of the glass tubes. Further, if the fluid within the glass tubes is corrosive or otherwise toxic and is at a higher pressure relative to the outside environment, then it may create health or other problems if is escapes from the joint.
Another method of improving the impermeability of the ball joint is to provide a coupling that exerts axial pressure on the joint, and thus tends to hold the ball within the socket. Typically, a coupling is used in combination with a seal, as this tends to provide the highest joint integrity. The coupling works by engaging one portion of the coupling on the back of the socket and engaging another portion of the coupling on the back of the ball. These pieces are then drawn toward each other and held together with some sort of mechanism, such as a single spring and fulcrum set, or a single set screw and fulcrum.
One design problem that exists with couplings is how to get the coupling pieces around the glass tubes. If the coupling is placed on the glass tubes by sliding it over either the ball or the socket from their free ends, then the hole in the coupling is too large to engage the back of the ball or socket and draw them toward each other. This is not a problem when the glass tubes to be joined are short and not attached to anything else at their other ends. In this case, the coupling halves can merely be slid over the free ends of the tubes, which typically have a smaller diameter than the ball and socket. Thus, in such a case, the hole in each coupling piece can be just large enough to fit over the glass tubing, and small enough so that each piece will engage the backs of the ball and socket.
However, in most applications either the glass tube is so long as to make it inconvenient to slip the coupling over the free end of the glass tube, or the glass tube is connected to something else, such as a plate, bell jar, or larger diameter tube, over which the coupling piece cannot be placed.
One method of overcoming this problem is to make each of the coupling halves in the shape of a "c." The open side of the c is just large enough to slip over the smaller diameter of the glass tube from the side rather than from one end or the other, and the rest of the c is used to apply pressure to the backs of the ball and socket. Unfortunately, the coupling cannot apply any pressure to the backs of the ball or socket in the cutout portion of the c, and the part which draws the two coupling pieces together is typically located at a single point opposite the cutout portion of the c. Thus, these prior art c-couplings do not exert uniform axial loading on the ground glass joint.
This situation tends to create a condition in the joint where it may leak due to the unevenly applied forces. While this may not be a problem in some applications, in other applications, such as in the semiconductor manufacturing industry where ground glass ball and socket joints are used on reaction vessels such as diffusion tubes, even small contaminants can create a big problem.
What is needed therefore, is a system for more uniformly axially loading a ground glass joint, and a system for containing toxins that may escape from or enter through compromised ground glass joints.